Nearly 17 years ago, the New Yorker ran what is probably still the best-known cartoon about the world wide web. It featured two dogs sat at a computer, with one pawing the keyboard while saying to his chum, "On the internet, nobody knows you're a dog."

The notion that the internet is full of barefaced liars remains a popular one. And with good reason: dating sites allow Devito-esque men to pass themselves off as Schwarzenegger, while teens on MySpace Photoshop away their acne. And then there are the internet predators, such as Peter Chapman, convicted this month for murdering Ashleigh Hall, a 17-year-old girl he befriended online.

But for all the horror stories, there remains little hard evidence of how far the 700 million-plus users of MySpace, Facebook and other social-networking sites adopt a different persona online. So Mitja Back, a psychologist at Johannes Gutenberg-University of Mainz, set out to get some. And he eventually found that internet profiles are more truthful than you might think.

Back and his colleagues got 236 American and German students with Facebook accounts to fill in personality questionnaires that gave a picture of their openness, extroversion and other traits. Independent observers then scrutinised the respondents' Facebook pages, nosing through the photos, the messages and the listed interests. And they found a remarkable degree of overlap between how students projected themselves on the internet and how they saw themselves in real life. Outgoing teenagers didn't pretend to be sullen online; introverts didn't pose as the life and soul of the party.

Two caveats should be made here. Back and his team only surveyed those between 17 and 22 years old, and were purely interested in broad personal characteristics. But for Back, the study suggests people behave more honestly online than they are given credit for. Which is something to think about, next time you come across a Facebook page for an old friend who appears to have made a million and kept all their hair.